The disclosure of Japanese Patent Application No. HEI 11-39921 filed on Feb. 18, 1999 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the invention
The present invention relates to a gas separator for use in a fuel cell, a fuel cell, and a method for distributing gas in a fuel cell. More particularly, the invention relates to a fuel cell separator which is provided between adjacent unit cells in a fuel cell formed of a stacked plurality of unit cells, and which forms a fuel gas passage and an oxidative gas passage, together with adjacent members and separates a fuel gas and an oxidative gas from each other, a fuel cell incorporating the separator, and a method for distributing gas in the fuel cell.
2. Description of the Related Art
A fuel cell gas separator is a member that constitutes a fuel cell stack formed of a stacked plurality of unit cells, and that has a gas impermeability sufficiently high to prevent mixture of a fuel gas and an oxidative gas that are supplied to adjacent unit cells. A typical fuel cell gas separator has a rib-like surface structure having recessed and protruding portions forming passages for the fuel gas and the oxidative gas (this type of gas separator is often termed a xe2x80x9cribbed inter-connectorxe2x80x9d). When incorporated in a fuel cell stack, fuel cell separators form fuel gas or oxidative gas passages (in-cell gas passages) between their rib-like surface structures and adjacent members (gas diffusion layers).
In addition to the rib-like surface structure for forming a gas passage, a typical fuel cell separator has a predetermined hole structure. If unit cells provided with such gas separators are stacked to form a fuel cell stack, the hole structures of adjacent gas separators meet so as to form gas manifolds that extend through the fuel cell stack in a stacking direction. The manifolds convey the fuel gas or the oxidative gas supplied into the fuel cell from an external device to distribute the gas to the individual unit cells, and collect a waste fuel gas or a waste oxidative gas resulting from electrochemical reactions that occur in the individual unit cells to discharge the waste gas to the outside of the fuel cell. For these functions, the gas manifolds are connected to a gas passage in each unit cell (that is, the in-cell oxidative gas passage or in-cell fuel gas passage of each unit cell), so that the gas can flow between the gas manifolds and each in-cell gas passage.
FIG. 18 illustrates the construction of a known fuel cell gas separator in a plan view. A separator 930 has a hole structure, that is, four holes 940, 942, 950, 952 formed near the periphery of the separator 930. When a plurality of unit cells, each including a separator 930, are stacked to form a fuel cell, corresponding holes of adjacent separators 930 meet so as to form four manifolds extending through the fuel cell. Specifically, these four manifolds are: an oxidative gas supply manifold for distributing the oxidative gas supplied from outside, into each in-cell oxidative gas passage; an oxidative gas discharge passage for collecting the waste oxidative gas from each in-cell oxidative gas passage and conveying the gas to the outside of the fuel cell; a fuel gas supply manifold for distributing the fuel gas supplied from outside, into each in-cell fuel gas passage; and a fuel gas discharge passage for collecting the waste fuel gas from each in-cell fuel gas passage and conveying the gas to the outside of the fuel cell.
A recessed portion 990 connecting the hole 940 and the hole 942 is formed in a surface of the separator 930 as shown in FIG. 18. The opposite surface of the separator 930 is provided with a recessed portion (not shown) connecting the hole 950 and the hole 952. Each recessed portion has a serpentine groove structure with two turns. When cell component members including separators 930 are stacked to form a fuel cell, the recessed portions of the separators 930 form, together with the members adjacent to the separators 930, in-cell gas passages. The recessed portion 990 connecting the holes 940 and 942 of each separator 930 forms an in-cell gas passage for the oxidative gas. The recessed portion connecting the holes 950 and 952 of each separator 930 forms an in-cell gas passage for the fuel gas. Therefore, the oxidative gas supplied into the fuel cell is conveyed through the oxidative gas supply manifold formed by the holes 940 of the separators 930, and distributed into the oxidative gas passage formed in each unit cell where the gas is used for the electrochemical reaction. After that, the waste gas flows out into the oxidative gas discharge manifold formed by the holes 942 of the separators 930, whereby the gas is discharged to the outside of the fuel cell. Similarly, the fuel gas supplied into the fuel cell is conveyed through the fuel gas supply manifold formed by the holes 950 of the separators 930, and distributed into the fuel gas passage formed in each unit cell, where the gas is used for the electrochemical reaction. After that, the waste gas flows out into the fuel gas discharge manifold formed by the holes 952 of the separators 930, whereby the gas is discharged to the outside of the fuel cell.
Since the recessed portion in each of the opposite surfaces of the separator 930 shown in FIG. 18 has a serpentine shape having two turns, the in-cell gas passage formed by each recessed portion has a reduced cross-sectional area in comparison with in-cell gas passages having no turns. Therefore, the gas flow velocity at a given location in each in-cell gas passage is increased, so that the gas flowing through the in-cell gas passage becomes well stirred and diffused. In such a well-stirred condition, hydrogen or oxygen in the gas (the fuel gas or the oxidative gas) is more likely to contact a catalyst layer provided on an electrode, so that the gas utilization rate in the electrochemical reactions increases.
A recessed structure formed in a surface of a fuel cell gas separator other than the recessed structure shown in FIG. 18 is proposed (in, for example, Japanese Patent Application Laid-open No. HEI 7-263003), in which a plurality of recessed portions, each having a serpentine shape with two turns as described above, are formed parallel in a surface of a separator, and gas is supplied to and discharged from the recessed portions via a gas introducing hole and a gas discharging hole that form a gas supply manifold and a gas discharge manifold.
However, in the fuel gas cell separators as illustrated in FIG. 18 or as described in the aforementioned laid-open patent application, each in-cell gas passage is provided with only one hole for introducing gas thereto (the hole 940 or 950 in FIG. 18) and only one hole for discharging gas therefrom (the hole 942 or 952 in FIG. 18), so that the flow of gas distributed to the individual unit cells of a fuel cell is likely become non-uniform or unequal. For example, water which is present as a result of the electrochemical reactions or the like may condense in a gas passage and may reside in an in-cell gas passage or near a junction between an in-cell gas passage and a gas manifold. If this happens, residing condensed water provides a resistance to gas flow, thereby impeding smooth flow of gas. If the gas supply condition deteriorates in this manner in a unit cell, sufficient progress of the electrochemical reactions in the unit cell is hindered. This may decrease the output voltage of the unit cell. In this manner, the output voltage varies among the unit cells of the entire fuel cell and, therefore, the performance of the fuel cell deteriorates.
Water condensation that may occur in a gas passage will be described. Condensation in the oxidative gas in a passage is attributed to water produced on a cathode side by an electrochemical reaction. The electrochemical reactions that occur in each unit cell of a polymer electrolyte fuel cell are shown below.
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1)
1/2O2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
H2+1/2O2xe2x86x92H2Oxe2x80x83xe2x80x83(3)
Equation (1) expresses a reaction that occurs at the anode in a fuel cell. Equation (2) expresses a reaction that occurs at the cathode. Equation (3) expresses a combined reaction that occurs in the entire fuel cell. As indicated above, water is produced at the cathode side as the cell reaction progresses in the polymer electrolyte fuel cell. The amount of water thus produced at the cathode side normally evaporates into the oxidative gas, and is discharged together with the oxidative gas to the outside of the fuel cell. However, if the amount of water produced is excessively large, or if a low-temperature region exits locally in the oxidative gas passage, an amount of water produced may condense and reside in the gas passage.
At the anode side, no water is produced by the electrochemical reaction. Normally, however, the fuel gas to be supplied to the anode is moisturized before being supplied to the fuel cell. Protons produced by the reaction expressed by equation (1) at the anode side hydrate with water molecules, and migrate in the form of hydrate through a solid electrolyte membrane toward the cathode side, so that a water-short condition occurs at the anode side. If the solid electrolyte dries, the electrical conductivity of the solid electrolyte decreases. Therefore, in a normal construction, the fuel gas is moisturized before being supplied to the fuel cell in order to prevent the solid electrolyte membrane from drying. In this manner, water vapor added to the fuel gas may condense in a fuel gas passage as described above. If water condenses and resides in an oxidative gas passage or a fuel gas passage so that the gas supply condition in some unit cells deteriorates as described above, the performance of the entire fuel cell may deteriorate.
The problem of variation in the output voltage among the unit cells of a fuel cell may be caused not only by the aforementioned water condensation, but also by a low precision in forming the fuel cell gas separators. If the forming precision of the recessed structure in a surface of a separator is low, that is, if there are variations in the depth of the recessed portions of the separators, the flow resistance to gas flow through in-cell gas passages varies among the unit cells, so that the amount of gas supplied varies among the unit cells. Therefore, if such low-precision separators are used in a fuel cell, the low forming precision of the separators causes variation in the output voltage among the unit cells, so that the performance of the entire fuel cell may deteriorate.
Accordingly, it is an object of the present invention to solve the aforementioned problems, that is, to prevent a reduction in the cell performance caused by non-uniform gas flow rates in the unit cells.
To achieve the aforementioned and other objects, one aspect of the invention provides a fuel cell formed of a stack of a plurality of unit cells. The fuel cell includes a gas passage provided in each unit cell, the gas passage conveying a gas to substantially the entire unit cell, a gas supply manifold that distributes the gas supplied to the fuel cell to gas passages of each unit cell, a gas discharge manifold that collects the gas from the gas passage of each unit cell and conveys the gas to outside the fuel cell, and a gas transit manifold extending in a unit cell-stacking direction and intersecting the gas passage of each unit cell, the gas transit manifold connecting the gas passages of the unit cells in communication.
Another aspect of the invention provides a method for distributing a gas in a fuel cell formed by stacking a plurality of unit cells. The method includes the steps of: distributing a supply of the gas to an in-cell gas passage formed in each unit cell, via a supply manifold formed in the fuel cell; causing an amount of the gas distributed to each in-cell gas passage to flow through the in-cell gas passage; causing at least a portion of the amount of the gas distributed to each in-cell gas passage to flow via a transit manifold which extends in a unit cell-stacking direction and which intersects each in-cell gas passage; causing the gas to flow out of each in-cell gas passage; and causing the gas from each unit cell to gather in a gas discharge manifold formed in the fuel cell and to flow out of the fuel cell.
In the fuel cell and the gas distributing method of the invention, the gas flows via the transit manifold when flowing through each in-cell gas passage. Therefore, if any unit cell of the fuel cell undergoes deterioration of the gas supply condition and therefore a reduction in the output voltage, the invention is able to prevent deterioration of the performance of the entire fuel cell. More specifically, if the passage resistance to the inflow of the gas into the in-cell gas passage of a unit cell increases due to, for example, condensed water residing therein or the like, so that the gas supply condition deteriorates, a sufficient amount of the gas can be supplied into a downstream-side in-cell gas passage because the flow of the gas from the upstream-side in-cell gas passage is supplemented when the gas flows in the transit manifold between the upstream and downstream-side in-cell gas passages. Therefore, even if condensed water resides in a unit cell, deterioration of the gas supply condition does not prevail in the entire unit cell.
Furthermore, in the fuel cell and the gas distributing method of the invention, since the gas flows via the transit manifold when flowing through each in-cell gas passage, the gas flow rates in the in-cell gas passages of the entire fuel cell can be substantially equalized. Since the in-cell gas passages communicate with one another via the transit manifold and the flows of the gas from the in-cell gas passages merge in the transit manifold, the gas flow rates become substantially equalized in the transit manifold even if the gas flow rates in in-cell gas passages upstream of the transit manifold significantly vary. In a typical fuel cell, the gas flow rates in the in-cell gas passages exhibit a predetermined gradient in the direction of the flow of the gas supplied from the outside and discharged to the outside (the direction of the flow of the gas in the gas discharge manifold). However, if the gas flow rates in the in-cell gas passages in the unit cells are substantially equalized as described above, the aforementioned gradient decreases so that each unit cell of the fuel cell receives the gas at a sufficiently high gas flow rate. Therefore, a high rate of the electrochemical reaction in each cell can be maintained.
According to the invention, the transit manifold may be provided in a plural number. Provision of a plurality of transit manifolds reduces the influence of hindrance of gas supply in a unit cell caused by water condensation or the like, and further equalizes the gas flow rates in the in-cell gas passages.
A still another aspect of the invention provides a separator for use in a fuel cell formed by stacking a plurality of unit cells. The separator includes at least a first hole portion, a second hole portion and a third hole portion for each forming a portion of a gas manifold of the fuel cell, the first hole portion, the second hole portion and the third hole portion including a first hole, a second hole and a third hole, respectively, which extend through a thickness of the separator, and a recessed portion for forming a gas passage, the recessed portion extending in a surface of the separator between the first hole and the second hole, via at least the third hole.
The separator of the invention makes it possible to form a fuel cell as described above. That is, the use of separators as described above makes it possible to form a fuel cell that has a reduced danger that the gas supply condition may deteriorate in a unit cell so as to reduce the output voltage of the unit cell and reduce the performance of the entire fuel cell. Furthermore, the use of separators as described above makes it possible to form a fuel cell which substantially equalizes the gas flow rates in the in-cell gas passages of the fuel cell and which secures a sufficiently high gas flow rate in each unit cell so that a high level of the electrochemical reaction in each cell can be maintained.
The recessed portion formed in a surface of the separator does not need to have a flat recessed surface. The recessed portion may also be provided with protrusions protruding from its surface. The recessed portion needs merely to extend in a surface of the separator between the first hole and the second hole, via at least the third hole.